Encapsulated sulfur cathodes for rechargeable lithium batteries

ABSTRACT

A method of forming a sulfur-based cathode material includes: 1) providing a sulfur-based nanostructure; 2) coating the nanostructure with an encapsulating material to form a shell surrounding the nanostructure; and 3) removing a portion of the nanostructure through the shell to form a void within the shell, with a remaining portion of the nanostructure disposed within the shell.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/404,058, filed on Jan. 11, 2017, which is a continuation of U.S.patent application Ser. No. 13/612,493, filed on Sep. 12, 2012, whichclaims the benefit of U.S. Provisional Application Ser. No. 61/533,740,filed on Sep. 12, 2011, and the benefit of U.S. Provisional ApplicationSer. No. 61/693,677, filed on Aug. 27, 2012, the disclosures of whichare incorporated herein by reference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contractDE-AC02-765F00515 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND

Rechargeable batteries with high specific energy are desirable forsolving imminent energy and environmental issues. Lithium-ion batterieshave one of the highest specific energy among rechargeable batteries,but state-of-the-art technology based on intercalation mechanism has atheoretical specific energy of about 400 Wh/kg for both LiCoO₂/graphiteand LiFePO₄/graphite systems. To achieve higher specific energy, newmaterials in both the cathode and anode are desired. Despite significantprogress in the development of high capacity anode materials such asnanostructured silicon, the relatively low charge capacity of cathodesremains a limiting factor for commercializing rechargeable batterieswith high specific energy. Current cathode materials, such as transitionmetal oxides and phosphates, typically have an inherent limit of about300 mAh/g. On the other hand, sulfur-based cathodes have a theoreticalcapacity of about 1,673 mAh/g. Although its voltage is about 2.2 V vsLi/Li⁺, which is about 60% of conventional lithium-ion batteries, thetheoretical specific energy of a lithium-sulfur cell is about 2,600Wh/kg, which is about five times higher than a LiCoO₂-graphite system.Sulfur also has many other advantages such as low cost and non-toxicity.However, the poor cycle life of lithium-sulfur batteries has been asignificant hindrance towards its commercialization. The fast capacityfading during cycling may be due to a variety of factors, including thedissolution of intermediate lithium polysulfides (e.g., Li₂S_(x), 4≤x≤8)in the electrolyte, large volumetric expansion of sulfur (about 80%)during cycling, and the insulating nature of Li₂S. In order to improvethe cycle life of lithium-sulfur batteries, the dissolution ofpolysulfides is one of the problems to tackle. Polysulfides are solublein the electrolyte and can diffuse to the lithium anode, resulting inundesired parasitic reactions. The shuttle effect also can lead torandom precipitation of Li₂S₂ and Li₂S on the positive electrode, whichcan change the electrode morphology and result in fast capacity fading.

Other approaches have been attempted to address material challenges ofsulfur, such as surface coating, conductive matrix, improvedelectrolytes, and porous carbon. For example, graphene/polymer coatinghas been shown to yield a smaller capacity decay. Porous carbon isanother approach to trap polysulfides and provide conductive paths forelectrons. Nevertheless, in the case of porous carbon, for example, alarge surface area of sulfur can still be exposed to the electrolyte,which exposure can cause undesired polysulfide dissolution. Moreover,lesser emphasis has been placed on dealing with the large volumeexpansion of sulfur during lithiation. This volume expansion of sulfurcan cause a surrounding material, such as a coating, to crack andfracture, rendering the surrounding material ineffective in trappingpolysulfides.

It is against this background that a need arose to develop thesulfur-based cathodes and related methods and electrochemical energystorage devices described herein.

SUMMARY

Embodiments of the invention relate to improved sulfur-based cathodesand the incorporation of such cathodes in electrochemical energy storagedevices, such as batteries and supercapacitors.

Sulfur has a high specific capacity of about 1,673 mAh/g as lithiumbattery cathodes, but its rapid capacity fading due to polysulfidedissolution presents a significant challenge for practical applications.Certain embodiments provide a hollow carbon nanofiber-encapsulatedsulfur cathode for effective trapping of polysulfides and exhibitinghigh specific capacity and excellent electrochemical cycling of batterycells. The hollow carbon nanofiber arrays are fabricated using an anodicaluminum oxide (“AAO”) template through thermal carbonization ofpolystyrene. The AAO template also facilitates sulfur infusion into thehollow fibers and substantially prevents sulfur from coating onto theexterior carbon wall. The high aspect ratio of carbon fibers provides adesirable structure for trapping polysulfides, and the thin carbon wallallows rapid transport of lithium ions. The dimension and shape of thesenanofibers provide a large surface area per unit mass for Li₂Sdeposition during cycling and reduce pulverization of active electrodematerials due to volumetric expansion. In some embodiments, a stabledischarge capacity of at least about 730 mAh/g can be observed at C/5rate after 150 cycles of charge/discharge. The introduction of LiNO₃additive to the electrolyte can improve the coulombic efficiency to atleast about 99% at C/5 rate. The hollow carbon nanofiber-encapsulatedsulfur structure can be useful as a cathode design for rechargeablelithium-sulfur batteries with high specific energy, such as at leastabout 500 Wh/kg, at least about 700 Wh/kg, at least about 900 Wh/kg, atleast about 1,100 Wh/kg, at least about 1,300 Wh/kg, at least about1,500 Wh/kg, at least about 1,700 Wh/kg, at least about 1,900 Wh/kg, orat least about 2,100 Wh/kg, and up to about 2,300 Wh/kg, up to about2,400 Wh/kg, up to about 2,500 Wh/kg, or up to about 2,600 Wh/kg.

Other embodiments provide substantially monodisperse,polymer-encapsulated hollow sulfur nanoparticles, presenting a rationaldesign to address various materials challenges. Some embodimentsdemonstrate high specific discharge capacities of at least about 1,179mAh/g, at least about 1,018 mAh/g, and at least about 990 mAh/g at C/10,C/5, and C/2 rates, respectively. Excellent capacity retention can beattained, with at least about 80.3% retention after 500 cycles and atleast about 60% retention after 1,000 cycles at C/2 rate. Together withthe high abundance of sulfur, embodiments provide a room-temperature,one-stage aqueous solution synthesis, which is highly scalable formanufacturing of low-cost and high-energy batteries.

Further embodiments demonstrate the design of a sulfur-TiO₂ yolk-shellnanoarchitecture with internal void space for stable and prolongedcycling over 1,000 charge/discharge cycles in lithium-sulfur batteries.Compared to bare sulfur and sulfur-TiO₂ core-shell nanoparticles, theyolk-shell nanostructures can exhibit high capacity retention due to thepresence of sufficient empty space to accommodate the volume expansionof sulfur, resulting in a structurally intact TiO₂ shell to mitigateagainst polysulfide dissolution. Using the yolk-shell nanoarchitecture,an initial specific capacity of at least about 1,030 mAh/g at C/2 rateand a Coulombic efficiency of at least about 98.4% over 1,000 cycles canbe achieved. Moreover, the capacity decay after 1,000 cycles can be0.033% or lower per cycle.

Other aspects and embodiments of the invention are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict the invention to any particular embodiment but aremerely meant to describe some embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof the invention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1. Schematic of design and fabrication process of hollow carbonnanofiber-encapsulated sulfur cathode structure. (a) The designprinciple showing the high aspect ratio of the carbon nanofiber foreffective trapping of polysulfides and (b) the fabrication process ofcathode structure. (c) Digital camera images showing the contrast of AAOtemplate before and after carbon coating and sulfur infusion.

FIG. 2. Schematic of sulfur nanoparticle with empty space and inside anouter shell, illustrating inward expansion during lithiation toaccommodate volume expansion and confinement of polysulfides by theshell.

FIG. 3A. Schematic of yolk-shell morphology to provide internal voidspace to accommodate volume expansion of sulfur during lithiation,resulting in structural intact shell for effective trapping ofpolysulfides.

FIG. 3B. Schematic of multi-yolk-shell morphology.

FIG. 4. Schematic of a battery including an encapsulated sulfur cathode.

FIG. 5. Scanning electron microscopy (“SEM”) characterizations of hollowcarbon nanofiber-encapsulated sulfur. (a) AAO template after carboncoating. (b) Carbon nanofiber-encapsulated sulfur after etching away AAOtemplate. (c) Cross-sectional image of hollow carbon/sulfur nanofiberarrays and elemental mapping of carbon (d) and sulfur (e) of FIG. 5 c.

FIG. 6. Transmission electron microscopy (“TEM”) characterizations ofhollow carbon nanofiber-encapsulated sulfur. (a) Bright field TEM imageof an individual nanofiber. The jagged line represents counts of sulfursignal along the dashed horizontal line. (b) Dark field scanning TEMimage (up) and energy-dispersive X-ray spectroscopy mapping of sulfur(down) of the nanofiber. (c) Zoom-in image of another sulfur-filledcarbon nanofiber, showing the thin carbon wall. (d) The correspondingaverage energy-dispersive X-ray spectrum obtained from the nanofiber in(c). Scale bars in FIGS. 6a and b are both 500 nm.

FIG. 7. Auger electron spectroscopy of the AAO/carbon template filledwith sulfur, before and after sputtering with Ar ions. (a) SEM image ofthe top view of carbon coated AAO template after infusion of sulfur. Thescale bar is 200 nm. Elemental mapping of sulfur (b) before Arsputtering, (c) 1.5 hours after Ar sputtering, (d) 4.5 hours after Arsputtering and (e) 7.5 hours after sputtering. The sputtering rate isabout 3.3 μm/hour.

FIG. 8. Electrochemical performance of the carbon nanofiber-encapsulatedsulfur cathode. (a) Typical charge/discharge voltage profiles at C/5 andC/2. (b) Cycle life at C/5 and C/2, as compared to a control sample inwhich the AAO was not etched away. The voltage range is 1.7-2.6 V vsLi/Li⁺.

FIG. 9. Electrochemical performance of the carbon nanofiber-encapsulatedsulfur electrode in electrolyte with LiNO₃ additive. (a) Capacities forcharge/discharge cycling at C/5. (b) Comparison of coulombicefficiencies for samples with and without LiNO₃ additive in theelectrolyte, for cycling at C/5 and C/2.

FIG. 10. SEM images of the two sides of AAO template.

FIG. 11. Raman spectra of four samples: pure sulfur, carbon coated AAOtemplate, hollow carbon nanofiber-encapsulated sulfur, and pure AAOtemplate. No noticeable sulfur signal was detected in the hollow carbonnanofiber-encapsulated sulfur.

FIG. 12. Comparison of the X-ray diffraction spectra for pristine sulfurand hollow carbon nanofiber-encapsulated sulfur. Inset is the zoom-inimage of the encapsulated sulfur between 22° and 24°

FIG. 13. X-ray diffraction spectra of AAO template after carbon coatingat 750° C. and 780° C.

FIG. 14. Charge/discharge voltage profiles of hollow carbonnanofiber-encapsulated sulfur at C/10 and C/5 rates.

FIG. 15. Fabrication, characterization and lithiation ofpolymer-encapsulated hollow sulfur nanoparticles. (a) Schematic of theformation mechanism for polymer-encapsulated hollow sulfurnanoparticles. (b) SEM image of the as-prepared polymer-encapsulatedhollow sulfur nanoparticles. (c) SEM image of the hollow sulfurnanoparticles after washing them with water to remove the polymer on theparticle surface. Inset in (c): TEM image of an individual hollow sulfurnanoparticle. (d) Schematic diagram illustrating the subliming processof the polymer-encapsulated hollow sulfur nanoparticles. (e-g) SEMimages of the sulfur nanoparticles (e) before, (f) during, and (g) aftersulfur sublimation, respectively. (h) X-ray photoelectron spectroscopy(“XPS”) spectra of polymer-encapsulated hollow sulfur nanoparticles (toptrace) and pure elemental sulfur (bottom trace). (i, j) Typical SEMimages of sulfur nanoparticles on a conducting carbon-fiber paper (i)before and (j) after lithiation. The particles after lithiation weremarked with circles in (j). (k) A comparison of the size distribution ofsulfur nanoparticles before and after lithiation.

FIG. 16. Electrochemical characteristics of polymer-encapsulated hollowsulfur nanoparticles. (a) Typical discharge-charge voltage profiles ofcells made from the polymer-encapsulated hollow sulfur nanoparticles atdifferent current rates (C/10, C/5 and C/2, 1 C=1673 mA/g) in thepotential range of 2.6-1.5 V at room temperature. (b) Cyclingperformance and Coulombic efficiency of the cell at a current rate ofC/5 for 300 cycles. (c) Cycling performance and Coulombic efficiency ofthe cell at a current rate of C/2 for 1,000 cycles. (d) Rate capabilityof the cell discharged at various current rates.

FIG. 17. Electrode thickness evaluation of a hollow sulfur nanoparticlecathode. (a) Schematic illustrating that an electrode thicknessundergoes little or no change owing to the inward expansion of each ofthe hollow sulfur nanoparticles upon lithiation. Typical SEM images ofthe cross-sections of the hollow sulfur nanoparticle cathode, showingthe thickness of (b) the pristine electrode and (c) the electrode after20 charge/discharge cycles (at fully discharged (lithiated) state). (d)A comparison of the thickness at 20 different locations for thecross-sections of the pristine electrode and the electrode after 20charge/discharge cycles.

FIG. 18. Size distribution of hollow sulfur nanoparticles, based oncounting over 200 particles from SEM images.

FIG. 19. Thermal gravimetric analysis (“TGA”) curve of as-preparedpolymer-encapsulated hollow sulfur nanoparticles recorded in the rangeof 40-400° C. in argon at a heating rate of about 2° C./min, showingthat an amount of elemental sulfur in the sample is about 70.4 wt %.Another curve is for a control sample of pure polymer powder under thesame experimental condition.

FIG. 20. Voltage profile of a pouch cell assembled in an argon-filledglovebox using a carbon-fiber paper with polymer-encapsulated hollowsulfur nanoparticles as cathode and lithium foil as anode. The pouchcell was discharged at a current rate of C/5 to 1.5 V, and then thevoltage was held at 1.5 V for about 18 h.

FIG. 21. Typical discharge/charge profiles of a cell made frompolymer-encapsulated hollow sulfur nanoparticles at 1 C rate.

FIG. 22. Synthesis and characterization of sulfur-TiO₂ yolk-shellnanostructures. (a) Schematic of the synthetic process which involvescoating of sulfur nanoparticles with TiO₂ to form sulfur-TiO₂ core-shellnanostructures, followed by partial dissolution of sulfur in toluene toachieve the yolk-shell morphology. (b) SEM image and (c) TEM image ofas-synthesized sulfur-TiO₂ yolk-shell nanostructures. Throughlarge-ensemble measurements, the average nanoparticle size and TiO₂shell thickness were determined to be about 800 nm and about 15 nm,respectively.

FIG. 23. Morphology of sulfur-TiO₂ yolk-shell nanostructures afterlithiation. (a-c) SEM images of sulfur-TiO₂ yolk-shell nanostructures(a) before and (b) after lithiation and (c) their respective particlesize distributions. (d) TEM image of a sulfur-TiO₂ yolk-shellnanostructure after lithiation, showing the presence of an intact TiO₂shell (highlighted by arrow). (e) Energy-dispersive X-ray spectrum andelectron energy loss spectrum (inset) of the nanostructure in (d),showing the presence of lithiated sulfur and TiO₂. The Cu peak arisesdue to the use of a copper TEM grid.

FIG. 24. Electrochemical performance of sulfur-TiO₂ yolk-shellnanostructures. (a) Charge/discharge capacity and Coulombic efficiencyover 1,000 cycles at 0.5 C. (b) Capacity retention of sulfur-TiO₂yolk-shell nanostructures cycled at 0.5 C, in comparison with baresulfur and sulfur-TiO₂ core-shell nanoparticles. (c) Charge/dischargecapacity and (d) voltage profiles of sulfur-TiO₂ yolk-shellnanostructures cycled at various C-rates from 0.2 C to 2 C. Specificcapacity values were calculated based on the mass of sulfur.

FIG. 25. (a) SEM image, (b) TEM image, and (c) Energy-dispersive X-rayspectrum of as-synthesized bare sulfur nanoparticles.

FIG. 26. (a) SEM image, (b) TEM image, and (c) Energy-dispersive X-rayspectrum of as-synthesized sulfur-TiO₂ core-shell nanoparticles. TheTiO₂ shell is not clearly visible in this case because it is relativelythin compared to the size of the entire nanoparticle.

FIG. 27. X-ray diffraction patterns of bare sulfur, sulfur-TiO₂core-shell nanoparticles, and sulfur-TiO₂ yolk-shell nanoparticles. Inthe sulfur-TiO₂ core-shell and yolk-shell particles, the diffractionpeaks of sulfur (marked with asterisks) were observed but not those ofTiO₂, indicating the amorphous nature of TiO₂.

FIG. 28. Voltage profile of a pouch cell assembled using sulfur-TiO₂yolk-shell nanoparticles on carbon-fiber paper as the working electrodeand lithium foil as the counter electrode. The cell was discharged at0.1 C to a voltage of 1.7 V, and the voltage was maintained for over 20h.

FIG. 29. SEM images of (a) bare sulfur and (b) sulfur-TiO₂ core-shellnanoparticles after lithiation, showing random precipitation ofirregularly-shaped Li₂S₂ and Li₂S particles on the electrodes due todissolution of lithium polysulfides into the electrolyte.

FIG. 30. TGA of bare sulfur, sulfur-TiO₂ core-shell nanoparticles, andsulfur-TiO₂ yolk-shell nanoparticles. The wt % of sulfur in these 3samples were determined to be about 99%, about 79%, and about 71%,respectively.

FIG. 31. Charge/discharge capacity and Coulombic efficiency ofsulfur-TiO₂ yolk-shell nanostructures in terms of mAh/g of the electrodemix over 1,000 cycles at 0.5 C.

FIG. 32. SEM images of the electrode cross-section of sulfur-TiO₂yolk-shell nanoparticles (a) before and (b) after 70 cycles at thevarious C-rates shown in FIG. 24c . (c) Their correspondingdistributions in electrode thickness, showing little change in thicknessbefore and after 70 cycles.

DETAILED DESCRIPTION Encapsulated Sulfur Cathodes

Embodiments of the invention relate to improved sulfur-based cathodematerials and the incorporation of such cathode materials inelectrochemical energy storage devices, such as batteries andsupercapacitors. Embodiments of the invention can effectively addressthe materials challenges of sulfur-based cathode materials thatotherwise can lead to rapid capacity fading in lithium-sulfur batteries.In some embodiments, the materials design and synthesis of sulfur-basedcathode materials can realize superior performance for high capacitysulfur-based cathodes. Lithium-sulfur batteries incorporating suchcathodes can show high specific discharge capacities and high capacityretention over long cycling. Together with the high abundance of sulfur,manufacturing of sulfur-based cathode materials can be carried out in ahighly scalable and low-cost manner.

Some embodiments relate to an encapsulating structure for a sulfur-basedelectrode having at least one or any combination or sub-combination ofthe following characteristics: 1) a largely or substantially closedstructure for efficient containment of a sulfur-based material (e.g.,one or more of elemental sulfur, a metal sulfide, and a metalpolysulfide); 2) a reduced surface area for sulfur-electrolyte contact;3) sufficient empty space to accommodate sulfur volumetric expansion toavoid or reduce pulverization of a sulfur-based active material; 4) ashort transport pathway for either, or both, electrons and Li ions toachieve high capacity at a high power rate; 5) a large conductivesurface area in contact with a sulfur-based material; and 6) a set ofsuitable electrolyte additives to passivate a lithium surface tominimize or reduce the shuttle effect.

Some embodiments implement a cathode structure with a sulfur-basedmaterial contained or disposed within largely or substantiallycontinuous, hollow encapsulating structures to mitigate againstdissolution of the sulfur-based material. The sulfur-based material caninclude one or more of elemental sulfur, a metal sulfide, a metalpolysulfide, or a mixture thereof. For example, the sulfur-basedmaterial can include one or more of elemental sulfur, Li₂S, Li₂S₂,Li₂S₃, Li₂S_(x) with 4≤x≤8, or a mixture thereof. The relatively thinwalls of the encapsulating structures can enhance the electricalconductivity of the cathode while allowing for the transfer of Li ionsthrough the walls, thereby also affording enhanced ionic conductivity.In conjunction, the walls of the encapsulating structures can serve aseffective barriers against polysulfide leakage and dissolution, whichbarriers are disposed between at least a portion of the sulfur-basedmaterial and an electrolyte. In such manner, the walls of theencapsulating structures can form a largely or substantially closedstructure for efficient containment of the sulfur-based material,thereby spatially separating or segregating the sulfur-based material asone or more active material domains in an interior of the encapsulatingstructures and apart from an electrolyte in an exterior of theencapsulating structures. In some embodiments, the presence of barriersand the separation of a sulfur-based material from an electrolyte by thebarriers can be demonstrated using, for example, microscope images asexplained in the Examples further below. In addition, the presence ofvoids or empty spaces inside the encapsulating structures allows forsulfur expansion during electrochemical cycling.

Referring to an embodiment of FIG. 1a , a hollow carbonnanofiber-encapsulated sulfur cathode 100 is provided, including asubstantially vertical array of hollow carbon nanofibers 102 partiallyfilled with a sulfur-based active material 104 (FIG. 1a ). The array ofhollow carbon nanofibers 102 can be an ordered or disordered array. AAOmembranes are used as templates for the fabrication of hollow carbonnanofibers 102, through a polystyrene carbonization process. The AAOmembranes serve both as a template for carbon nanofiber formation and abarrier to inhibit the sulfur-based material 104 from coating onto theexterior carbon fiber walls. In such manner, the sulfur-based material104 is selectively coated onto the inner surfaces of the hollownanofibers 102, with any gaps or spaces between the nanofibers 102substantially devoid of the sulfur-based material 104. By way ofexample, the nanofiber diameters (e.g., outer diameter) can range fromabout 200 nm to about 300 nm, while the length is up to about 60 μm ormore, corresponding to the AAO template structure. The sulfur-basedmaterial 104 is effectively contained in the high aspect ratio carbonnanofibers 102, and its contact with an electrolyte is limited to thetwo openings at the ends of the nanofibers 102. It is also contemplatedthat the ends of the nanofibers 102 can be capped to further reducecontact with the electrolyte, such as by depositing or otherwiseapplying a suitable material to the ends of the nanofibers 102. Thehollow structure provides a large space for sulfur expansion duringcycling. As lithium can readily penetrate the thin carbon wall, rapidionic transport is also possible. The one-dimensional nature ofconductive carbon allows facile transport of electrons and a large areafor contact with the sulfur-based material 104. It is also contemplatedthat the inner surfaces of the carbon nanofibers 102 can be chemicallymodified to facilitate coating of the sulfur-based material 104. Theseattributes of the hollow carbon nanofiber structure allow high specificcapacity and stable cycle life of the sulfur-based cathode 100 inlithium-sulfur batteries.

More generally for some embodiments, carbon nanofibers (or other typesof hollow, elongated encapsulating structures) can have an outer lateraldimension (e.g., an outer diameter, an outer lateral dimension along amajor axis, an averaged outer lateral dimension along a major axis and aminor axis, or another characteristic outer lateral dimension) in therange of about 10 nm to about 5 μm, such as about 20 nm to about 5 μm,from about 30 nm to about 2 μm, about 30 nm to about 1 μm, about 30 nmto about 900 nm, about 50 nm to about 800 nm, about 50 nm to about 700nm, about 50 nm to about 600 nm, about 50 nm to about 500 nm, about 100nm to about 500 nm, about 50 nm to about 400 nm, about 100 nm to about400 nm, or about 200 nm to about 300 nm, a longitudinal dimension (e.g.,a length or another characteristic longitudinal dimension) in the rangeof about 500 nm to about 500 μm, such as about 800 nm to about 400 μm,about 1 μm to about 300 μm, about 1 μm to 200 μm, about 1 μm to about150 μm, about 1 μm to about 100 μm, or about 10 μm to about 100 μm, andan aspect ratio (e.g., specified as a ratio of its longitudinaldimension and its outer lateral dimension) that is greater than about 1,such as at least or greater than about 5, at least or greater than about10, at least or greater than about 20, at least or greater than about50, at least or greater than about 100, at least or greater than about300, from about 2 to about 2,000, about 5 to about 1,000, about 10 toabout 900, about 10 to about 800, about 50 to about 700, about 50 toabout 600, about 50 to about 500, about 100 to about 500, about 100 toabout 400, or about 200 to about 400. Also, the carbon nanofibers (orother types of hollow, elongated encapsulating structures) can have aninner lateral dimension (e.g., an inner diameter, an inner lateraldimension along a major axis, an averaged inner lateral dimension alonga major axis and a minor axis, or another characteristic inner lateraldimension defining an internal volume to accommodate a sulfur-basedmaterial) that is at least about 10 nm, such as at least about 15 nm, atleast about 20 nm, at least about 30 nm, at least about 40 nm, at leastabout 50 nm, at least about 100 nm, at least about 150 nm, or at leastabout 200 nm, and up to an outer lateral dimension while accounting fora thickness of the walls of the carbon nanofibers. The walls of thecarbon nanofibers (or other types of hollow, elongated encapsulatingstructures) can be in the range of about 0.5 nm to about 100 nm, such asabout 1 nm to about 90 nm, about 1 nm to about 80 nm, about 1 nm toabout 70 nm, about 1 nm to about 60 nm, about 1 nm to about 50 nm, about1 nm to about 40 nm, about 1 nm to about 30 nm, about 5 nm to about 30nm, about 10 nm to about 30 nm, about 1 nm to about 20 nm, about 5 nm toabout 20 nm, about 10 nm to about 20 nm, about 1 nm to about 10 nm, orabout 5 nm to about 20 nm. The above specified values for dimensions,thicknesses, and aspect ratios can apply to an individual carbonnanofiber (or another type of hollow, elongated encapsulatingstructure), or can represent an average or a median value across apopulation of carbon nanofibers.

Diameters of carbon nanofibers (or other types of hollow, elongatedencapsulating structures) can be substantially constant or can varyalong the lengths of the nanofibers, such as in accordance with a poremorphology of an AAO template structure. Hollow, elongated structurescan be formed of other types of conductive materials in place of, or incombination with carbon, such as titanium oxide (doped or undoped) andother types of metal oxides. Examples of other hollow, elongatedstructures include hollow, metal nanofibers; hollow, metal oxidenanofibers; hollow, metal nitride nanofibers; hollow, metal sulfidenanofibers; and hollow, composite nanofibers. Hollow, elongatedstructures can be single-shelled or multi-shelled, with different shellsformed of the same material or different materials, and surfaces of thehollow, elongated structures can be smooth or rough. Hollow, elongatedstructures can be electrically conductive, ionically conductive (e.g.,with respect to one or more of Li ions, Na ions, K ions, Mg ions, Alions, Fe ions, and Zn ions), or both.

Advantageously, a sulfur-based material is selectively coated in awell-controlled and reproducible manner onto the inner surfaces ofhollow carbon nanofibers, instead of their exterior surfaces, andinstead of incorporation within walls of the carbon nanofibers. In suchmanner, the sulfur-based material can form one or more active materialdomains that are spaced apart from an electrolyte by the walls of thecarbon nanofibers, and exposure of the sulfur-based material to anelectrolyte can be reduced, thereby addressing the dissolution issue. Totackle this issue, a template-assisted method is used to fabricate acathode structure with sulfur selectively coated on the inner wall ofthe carbon fibers, as shown in FIG. 1b . AAO template (e.g., Whatman,pore size of about 200 nm, thickness of about 60 μm) is used as thetemplate for making hollow carbon nanofibers. Typically, about 120 mg ofAAO membrane is placed inside an alumina boat, and about 2 ml of about10 wt % polystyrene (“PS”) (or another suitable carbon-containingpolymer) suspended in dimethylformamide (“DMF”) (or another suitablesolvent) is dropped onto the template as the carbon precursor. Thecarbonization is performed by heating the AAO/PS/DMF mixture at about750° C. (or another suitable temperature, such as in the range of about500° C. to about 1,000° C.) for about four hours (or another suitabletime period, such as in the range of about 1 hour to about 10 hours)under a slow flow of N₂ gas. After cooling down, the carbon-coated AAOtemplate is loaded into a small glass vial, together with a controlledamount of about 1% sulfur solution in toluene (or another suitablesolvent). The sample is dried in a vacuum oven, before being heated upto about 155° C. (or another suitable temperature, such as in the rangeof about 100° C. to about 200° C.) and kept for about 12 hours (oranother suitable time period, such as in the range of about 5 hours toabout 20 hours) to ensure uniform sulfur diffusion into the carbonfibers. In this fabrication process, the AAO membrane not only providesa template for hollow carbon nanofiber formation, but also prevents ormitigates against sulfur from coating onto the external surface of thefiber wall. To remove the AAO template, the AAO/carbon nanofiber/Scomposite is immersed in about 2 M H₃PO₄ solution (or another suitableacidic solution) for about 10 hours (or another suitable time period,such as in the range of about 5 hours to about 20 hours). FIG. 1c showsdigital camera images of a pristine AAO template before (lighter shade)and after (darker shade) carbon coating and sulfur infusion, indicatingthat sulfur was absorbed into the hollow carbon fibers. Other types ofporous template structures can be used in place of, or in combinationwith, the AAO template structure.

Referring to another embodiment of FIG. 2, encapsulating structures 200are in the form of outer shells having a spherical or spheroidal shape.The encapsulating structures 200 can have an outer lateral dimension(e.g., an outer diameter, an outer lateral dimension along a major axis,an averaged outer lateral dimension along a major axis and a minor axis,or another characteristic outer lateral dimension) in the range of about10 nm to about 10 μm, such as about 10 nm to about 5 μm, about 50 nm toabout 2 μm, about 100 nm to about 1 μm, about 100 nm to about 900 nm,about 200 nm to about 800 nm, about 300 nm to about 700 nm, about 300 nmto about 600 nm, or about 400 nm to about 500 nm, and an aspect ratio(e.g., specified as a ratio of outer lateral dimensions along a majoraxis and a minor axis) that is less than about 5, such as no greaterthan about 4.5, no greater than about 4, no greater than about 3.5, nogreater than about 3, no greater than about 2.5, no greater than about2, no greater than about 1.5, or about 1. In some embodiments, thehollow, spheroidal encapsulating structures are largely or substantiallymonodisperse, such that at least about 50%, at least about 60%, at leastabout 70%, at least about 80%, at least about 90%, or at least about 95%of the hollow, spheroidal encapsulating structures are within one ormore of the ranges of dimensions specified above. Also, the hollow,spheroidal encapsulating structures can have an inner lateral dimension(e.g., an inner diameter, an inner lateral dimension along a major axis,an averaged inner lateral dimension along a major axis and a minor axis,or another characteristic inner lateral dimension defining an internalvolume to accommodate a sulfur-based material) that is at least about 10nm, such as at least about 15 nm, at least about 20 nm, at least about30 nm, at least about 40 nm, at least about 50 nm, at least about 100nm, at least about 150 nm, at least about 200 nm, at least about 250 nm,at least about 300 nm, at least about 350 nm, or at least about 400 nm,and up to an outer lateral dimension while accounting for a thickness ofthe walls of the encapsulating structures. The walls of theencapsulating structures can have a thickness in the range of about 1 nmto about 100 nm, such as about 5 nm to about 90 nm, about 10 nm to about80 nm, about 10 nm to about 70 nm, about 10 nm to about 60 nm, about 10nm to about 50 nm, about 10 nm to about 40 nm, or about 10 nm to about30 nm. As shown in FIG. 2, the hollow, spheroidal encapsulatingstructures are formed of a polymer, such as poly(vinyl pyrrolidone) oranother conductive polymer having polar groups, non-polar groups, orboth (e.g., an amphiphilic polymer). Hollow, spheroidal encapsulatingstructures can be formed of other types of conductive materials in placeof, or in combination with, a polymer, such as carbon, metals, titaniumoxide (doped or undoped), and other types of metal oxides, metalnitrides, and metal sulfides. Examples of other hollow, spheroidalstructures include hollow, metal shells; hollow, metal oxide shells;hollow, metal nitride shells; hollow, metal sulfide shells; and hollow,composite shells. Hollow, spheroidal encapsulating structures can beelectrically conductive, ionically conductive (e.g., with respect to Liions or other types of ions), or both.

As shown in FIG. 2, a sulfur-based material 202 is disposed within theencapsulating structures 200, with a void or an empty space disposedwithin an interior of each encapsulating structure 200. In theillustrated embodiment, the sulfur-based material 202 is provided asinner shells or as hollow nanoparticles, with a void or an empty spacedisposed within an interior of each hollow nanoparticle. In otherembodiments, a sulfur-based material 302 or 306 can be disposed withineach encapsulating structure 300 or 304 as one or more substantiallysolid nanoparticles or other types of substantially solid nanostructures(FIGS. 3A and 3B). The embodiment of FIG. 3A can be referred to ashaving a yolk-shell morphology, with the sulfur-based material 302corresponding to a “yolk” surrounded by an outer “shell” of theencapsulating structure 300, and the embodiment of FIG. 3B can bereferred to as having a multi-yolk-shell morphology, with thesulfur-based material 306 corresponding to multiple “yolks” surroundedby an outer “shell” of the encapsulating structure 304. Although twonanoparticles of the sulfur-based material 306 are shown in FIG. 3B,more or fewer nanoparticles of the sulfur-based material 306 can beincluded in the encapsulating structure 304, and the number ofnanoparticles can be substantially uniform or can vary across apopulation of the encapsulating structures 304.

More generally, sulfur-based nanoparticles (or other types ofnanostructures formed of a sulfur-based material) can have an outerlateral dimension (e.g., an outer diameter, an outer lateral dimensionalong a major axis, an averaged outer lateral dimension along a majoraxis and a minor axis, or another characteristic outer lateraldimension) that is at least about 1 nm, such as at least about 5 nm, atleast about 10 nm, at least about 15 nm, at least about 20 nm, at leastabout 30 nm, at least about 40 nm, at least about 50 nm, at least about100 nm, at least about 150 nm, at least about 200 nm, at least about 250nm, at least about 300 nm, at least about 350 nm, or at least about 400nm, and up to an inner lateral dimension of encapsulating structures,while leaving sufficient room for volume expansion during cycling. Asulfur-based nanoparticle can be largely or substantially solid, or canhave a void or an empty space disposed within an interior of, and atleast partially surrounded by, the sulfur-based nanoparticle. In someembodiments, sulfur-based nanoparticles have a spherical or spheroidalshape. Typically, a sulfur-based nanoparticle has an aspect ratio thatis less than about 5, such as no greater than about 4.5, no greater thanabout 4, no greater than about 3.5, no greater than about 3, no greaterthan about 2.5, no greater than about 2, no greater than about 1.5, orabout 1. Other types of sulfur-based nanostructures can be used in placeof, or in combination with, sulfur-based nanoparticles, such aselongated nanostructures having an aspect ratio that is at least about5, whether solid or hollow.

Referring, for example, to the embodiments of FIGS. 1 through 3B, anexterior of encapsulating structures can be largely or substantiallydevoid of a sulfur-based material. Also, the walls of the encapsulatingstructures can be largely or substantially devoid of the sulfur-basedmaterial. Stated in another way, the sulfur-based material can beselectively positioned in a well-controlled and reproducible mannerwithin an interior of the encapsulating structures, instead of theirexterior surfaces, and instead of infiltration or impregnation of thesulfur-based material within walls of the encapsulating structures. Insome embodiments, at a portion of a sulfur-based material is spatiallyseparated or segregated as one or more active material domains in aninterior of each encapsulating structure and apart from an electrolytein an exterior of the encapsulating structure, such as at least about10% (by weight or volume), at least about 20% (by weight or volume), atleast about 30% (by weight or volume), at least about 40% (by weight orvolume), at least about 50% (by weight or volume), at least about 60%(by weight or volume), at least about 70% (by weight or volume), atleast about 75% (by weight or volume), at least about 80% (by weight orvolume), at least about 85% (by weight or volume), at least about 90%(by weight or volume), at least about 95% (by weight or volume), or atleast about 98% (by weight or volume), and up to about 99% (by weight orvolume), or up to about 99.9% (by weight or volume), up to about 99.99%(by weight or volume), or more, relative to any remaining portion of thesulfur-based material on an exterior and within a wall of theencapsulating structure.

Still referring, for example, to the embodiments of FIGS. 1 through 3B,each encapsulating structure defines an internal volume, and asulfur-based material is disposed within the internal volume andoccupies less than 100% of the internal volume, thereby leaving a voidor an empty space inside the encapsulating structure to allow forexpansion of the sulfur-based material. In some embodiments, such as forthe case of the sulfur-based material is its substantially de-lithiatedstate, a ratio of the volume of the void inside the encapsulatingstructure relative to the volume of the sulfur-based material inside theencapsulating structure is in the range of about 1/20 to about 20/1,such as from about 1/10 to about 10/1, from about 1/10 to about 5/1,from about 1/10 to about 3/1, from about 1/10 to about 2/1, from about1/10 to about 1/1, from about 1/5 to about 3/1, from about 1/5 to about2/1, from about 1/5 to about 1/1, from about 1/3 to about 3/1, fromabout 1/3 to about 2/1, from about 1/3 to about 1/1, from about 1/2 toabout 3/1, from about 1/2 to about 2/1, from about 1/2 to about 1/1,from about 2/3 to about 3/1, from about 2/3 to about 2/1, or from about2/3 to about 1/1. In some embodiments, such as for the case of thesulfur-based material is its substantially de-lithiated state, thevolume of the void can be at least about 1/20 of the total internalvolume inside the encapsulating structure, such as at least about 1/10,at least about 1/5, at least about 1/3, at least about 1/2, or at leastabout 2/3, with a remainder of the internal volume inside theencapsulating structure taken up by the sulfur-based material. Theloading of the sulfur-based material within the encapsulating structurescan be controlled so that there is enough empty space for sulfur toexpand during lithiation. In some embodiments, a weight ratio of thesulfur-based material relative to a combined mass of the sulfur-basedmaterial and the encapsulating structures is in the range of about 1% toabout 99%, such as from about 5% to about 95%, from about 10% to about90%, from about 20% to about 90%, from about 30% to about 80%, fromabout 40% to about 80%, from about 50% to about 80%, or from about 60%to about 80%.

Electrochemical Energy Storages Including Encapsulated Sulfur Cathodes

The electrodes described herein can be used for a variety of batteriesand other electrochemical energy storage devices. For example, theelectrodes can be substituted in place of, or used in conjunction with,conventional electrodes for lithium-sulfur batteries or other types ofbatteries. As shown in an embodiment of FIG. 4, a resulting battery 400can include a cathode 402, an anode 404, and a separator 406 that isdisposed between the cathode 402 and the anode 404. The battery 400 alsocan include an electrolyte 408, which is disposed between the cathode402 and the anode 404. The cathode 402 can be an encapsulated sulfurcathode as described herein, and the anode 404 can be a lithium-basedanode, a silicon-based anode, a germanium-based anode, or anothersuitable anode.

Resulting batteries, such as the battery 400, can exhibit a maximumdischarge capacity at a current rate of C/10 (or at C/5, C/2, 1 C, oranother higher or lower reference rate and as evaluated relative toLi/Li⁺ or another counter/reference electrode) that is at least about400 mAh/g, such as at least about 500 mAh/g, at least about 600 mAh/g,at least about 700 mAh/g, at least about 800 mAh/g, at least about 900mAh/g, at least about 1,000 mAh/g, at least about 1,100 mAh/g, at leastabout 1,200 mAh/g, at least about 1,300 mAh/g, at least about 1,400mAh/g, or at least about 1,500 mAh/g, and up to about 1,670 mAh/g ormore, such as up to about 1,600 mAh/g or up to about 1,560 mAh/g.

Resulting batteries, such as the battery 400, also can exhibit excellentretention of discharge capacity over several cycles, such that, after100 cycles at a rate of C/10 (or at C/5, C/2, 1 C, or another higher orlower reference rate), at least about 50%, at least about 60%, at leastabout 70%, at least about 75%, at least about 80%, at least about 85%,or at least about 88%, and up to about 90%, up to about 95%, or more ofan initial, maximum, or other reference discharge capacity (e.g., at the14^(th) cycle) is retained. And, after 200 cycles at a rate of C/10 (orat C/5, C/2, 1 C, or another higher or lower reference rate), at leastabout 45%, at least about 55%, at least about 65%, at least about 70%,at least about 75%, at least about 83%, at least about 85%, or at leastabout 87%, and up to about 90%, up to about 95%, or more of an initial,maximum, or other reference discharge capacity (e.g., at the 14^(th)cycle) is retained. And, after 500 cycles at a rate of C/10 (or at C/5,C/2, 1 C, or another higher or lower reference rate), at least about40%, at least about 50%, at least about 60%, at least about 65%, atleast about 70%, at least about 75%, at least about 80%, or at leastabout 81%, and up to about 90%, up to about 95%, or more of an initial,maximum, or other reference discharge capacity (e.g., at the 14^(th)cycle) is retained. And, after 1,000 cycles at a rate of C/10 (or atC/5, C/2, 1 C, or another higher or lower reference rate), at leastabout 30%, at least about 40%, at least about 50%, at least about 55%,at least about 60%, at least about 63%, at least about 65%, or at leastabout 67%, and up to about 80%, up to about 85%, or more of an initial,maximum, or other reference discharge capacity (e.g., at the 14^(th)cycle) is retained.

Also, in terms of coulombic efficiency (e.g., an initial or a maximumcoulombic efficiency or one that is averaged over a certain number ofcycles, such as 100, 200, 500, or 1,000 cycles) at a rate of C/10 (or atC/5, C/2, 1 C, or another higher or lower reference rate), resultingbatteries, such as the battery 400, can have an efficiency that is atleast about 75%, at least about 80%, at least about 85%, at least about90%, at least about 95%, or at least about 98%, and up to about 99%, upto about 99.5%, up to about 99.9%, or more.

Moreover, in some embodiments, the effectiveness of containment of asulfur-based material and the structural integrity of encapsulationstructures can be assessed in terms of a weight percentage of sulfur(e.g., whether in elemental or another form) present in an electrolyteafter a certain number of cycles, relative to a total weight of sulfur(e.g., whether in elemental or another form) as initially included in acathode. In some embodiments, after 30 cycles at a rate of C/10 (or atC/5, C/2, 1 C, or another higher or lower reference rate), no greaterthan about 23% of sulfur is present in the electrolyte, such as nogreater than about 21% or no greater than about 19%. After 100 cycles ata rate of C/10 (or at C/5, C/2, 1 C, or another higher or lowerreference rate), no greater than about 25% of sulfur is present in theelectrolyte, such as no greater than about 23% or no greater than about21%. After 500 cycles at a rate of C/10 (or at C/5, C/2, 1 C, or anotherhigher or lower reference rate), no greater than about 30% of sulfur ispresent in the electrolyte, such as no greater than about 28% or nogreater than about 26%.

EXAMPLES

The following examples describe specific aspects of some embodiments ofthe invention to illustrate and provide a description for those ofordinary skill in the art. The examples should not be construed aslimiting the invention, as the examples merely provide specificmethodology useful in understanding and practicing some embodiments ofthe invention.

Example 1 Hollow Carbon Nanofiber-Encapsulated Sulfur

This example describe the synthesis of hollow carbonnanofiber-encapsulated sulfur electrode structures, including asubstantially vertical array of hollow carbon nanofibers filled withmelted sulfur. AAO membranes are used as templates for the fabricationof hollow carbon nanofibers, through a polystyrene carbonizationprocess.

Scanning electron microscopy (“SEM”) images of designed structures ofsome embodiments at different stages of fabrication are shown in FIG. 5.After carbon coating at about 750° C., substantially continuous hollowcarbon nanofibers are formed inside the AAO template (FIG. 5a ). Theouter diameters of the nanofibers are about 200 nm to about 300 nm,corresponding to the pore size of AAO template (FIG. 10). The weightgain after carbon coating was about 2% of the AAO template. FIG. 5bshows the image of hollow carbon nanofibers after sulfur infusion andAAO etching. Typically, the weight ratio of sulfur to carbon was 3:1 inthe final electrode structure, corresponding to about 75 vol % of sulfurcontent in the composite, although other suitable weight ratios arecontemplated, such as from about 1:1 to about 10:1 or about 1.5:1 toabout 5:1. The sulfur loading is controlled so that there is enough freespace (e.g., in the form of gaps or voids within and at least partiallyextending through the nanofibers after sulfur infusion) for sulfur toexpand during the formation of Li₂S. To confirm the presence of carbonand sulfur, energy-dispersive X-ray spectroscopy (“EDS”) mappings areperformed over the cross section of the whole carbon nanofiber array,with the corresponding SEM image in FIG. 5c . Carbon (FIG. 5d ) andsulfur (FIG. 5e ) signals are detected substantially uniformly over thewhole cross section, validating the structural design and indicatingthat sulfur was well distributed within the hollow carbon nanofibers.

Further evidence of sulfur containment within the carbon nanofiber wasprovided by transmission electron microscopy (“TEM”) images of someembodiments. FIG. 6a shows a hollow carbon nanofiber with sulfurencapsulated inside. Sulfur appears darker under TEM as it is heavierthan carbon. An EDS line-scan (dashed line) across the carbon nanofiberfurther confirms the presence of sulfur. The spectrum represents thecounts of sulfur signal along the dashed line. The spectrum shows thatsulfur is present inside the hollow carbon nanofibers, but not outside.This is also verified by the sulfur EDS mapping in FIG. 6b . The fullEDS spectrum over the whole tube (FIG. 6d ) shows the carbon and sulfurpeaks but not any aluminum signal, indicating that there is little or noalumina residue left from the AAO template. The zoom-in image (FIG. 6c )of another carbon nanofiber shows the fiber wall has a small thicknessof about 8 nm to about 9 nm, which is desirable in allowing fastkinetics of lithium ion diffusion.

Spatial distribution of sulfur inside the hollow carbon nanofiber arraysis further demonstrated by auger electron spectroscopy (“AES”) with Arion sputtering, according to some embodiments. FIG. 7a shows the topview SEM image of the nanofiber array, revealing the hexagonal packing.The elemental mapping of sulfur before sputtering (FIG. 7b ) also yieldsa similar hexagonal pattern, suggesting that sulfur is present in thehollow channels. FIG. 7c-e show the sulfur elemental mappings after 1.5hours, 4.5 hours and 7.5 hours of Ar ion sputtering respectively. Around25 μm of the sample is etched away after 7.5 hours of sputtering. Thevariation in the sulfur mapping patterns is due to the change in the AAOchannel morphology at different depths (FIG. 10). The hexagonal packingbecomes clearer at regions closer to the center of the hollow nanofiberarray. The AES mappings show that globally sulfur is well distributedfrom the top to deep inside the hollow carbon nanofibers. A variety ofother regular or irregular patterns are contemplated, such as squarepatterns, rectangular patterns, triangular patterns, octagonal patterns,and so forth.

The above characterizations show that hollow carbonnanofiber-encapsulated sulfur can be formed with the assist of AAOtemplate. To further understand the crystal structure of carbon andsulfur in the final structure, Raman spectroscopy and X-ray diffraction(“XRD”) are performed to study the as-fabricated sulfur electrode ofsome embodiments. The Raman measurement shows a typical spectrum ofpartially graphitized carbon, indicated by the G band (about 1600 cm⁻¹)and D band (about 1360 cm⁻¹) in FIG. 11. G band features the in-planevibration of sp² carbon atoms, and D band originates from the defects.The coexistence of the two bands indicates that the carbon was partiallygraphitized with some amount of defects and disorders. Furtheroptimizations can be implemented to reduce the amount of defects anddisorders. The absence of sulfur peak in the Raman spectrum of carbonnanofibers/S composite indicates that sulfur is well encapsulated withinthe carbon nanofibers.

XRD spectrum (FIG. 12) of the carbon/sulfur composites shows a weak peakat about 23.05°, corresponding to the strongest (222) peak oforthorhombic sulfur (PDF 00-001-0478). This indicates that sulfur in thehollow nanofiber was less crystalline, although further optimizationscan be implemented to enhance or otherwise adjust the degree ofcrystallinity. In some embodiments, there is no peak related tocrystalline Al₂O₃ phase in the XRD pattern, indicating that the AAOtemplate was still amorphous after carbonization at about 750° C. Thisis desirable for the etching of Al₂O₃. In contrast, AAO template heatedto about 780° C. can be more difficult to remove, and extra peaks appearin the XRD pattern, suggesting that AAO transformed into a crystallinephase (FIG. 13).

To evaluate the electrochemical performance of hollow carbonnanofiber-encapsulated sulfur, 2032-type coin cells are fabricatedaccording to some embodiments. The prepared sample is pressed ontoaluminum substrate (or another type of current collector) as the workingelectrode without any binder or conductive additives. Lithium is used asthe counter electrode. The electrolyte is about 1 M lithiumbis(trifluoromethanesulfonyl)imide (“LiTFSI”) in 1,3-dioxolane and1,2-dimethoxyethane (volume ratio of about 1:1), although otherLi-containing salts and organic solvents can be used. The typical massloading is about 1.0 mg sulfur/cm² (although another suitable massloading is contemplated, such as from about 0.1 mg sulfur/cm² to about10 mg sulfur/cm²), and the specific capacities are calculated based onthe sulfur mass alone in some embodiments.

The voltage profiles of hollow carbon nanofiber-sulfur composites atdifferent current rates are shown in FIG. 8a according to someembodiments. The discharge/charge profile of both C/5 and C/2 show thetypical two-plateau behavior of sulfur-based cathodes, corresponding tothe formation of long chain polysulfides (e.g., Li₂S_(x), 4≤x≤8) atabout 2.3 V and short chain Li₂S₂ and Li₂S at about 2.1 V. Moreover, thesecond plateau is substantially flat, suggesting a uniform deposition ofLi₂S with little kinetic barriers. It is also observed that the cyclingcapacity drop was small (about 5%) when current rate increases from C/10to C/5 after four cycles (FIG. 14), indicating good kinetics of theworking electrode. This can be attributed to the high quality of carbonand the thin carbon fiber wall, which significantly improved electronicand ionic transport at the cathode.

Cycling performance at C/5 and C/2 is presented in FIG. 8b , togetherwith that of the same carbon hollow fiber/S composite without removingAAO template, according to some embodiments. With AAO etched away, thecathode structure shows impressive capacity retention. At C/5, thereversible capacity is higher than about 900 mAh/g after 30 cycles ofcharge/discharge. A small decay of about 7% is observed in the next 30cycles, and the capacity is about 730 mAh/g after 150 cycles. Thedischarge capacity at C/2 also shows good stability, and the reversiblecapacity is about 630 mAh/g after 150 cycles. In the control sample inwhich the AAO template was not etched away, the electrode has a lowerstable capacity of about 380 mAh/g. Interestingly, the cycling stabilityof the non-etched sample is slightly better, as the capacity stabilizedafter 15 cycles of charge/discharge, and the decay is about 3% for thenext 30 cycles before leveling off. This shows that the removal of AAOtemplate can improve charge transfer through the sidewall of the carbonfibers to achieve high cycling capacity, but, at the same time, aluminacan potentially help trap polysulfides to improve the cycle life. Themechanical support provided by the AAO template can also enhance thestability of the cathode structure. Further optimizations of the etchingtime can provide a sulfur electrode with even higher specific capacityand more stable cycle life.

To further improve the battery performance, about 0.1 mol/L LiNO₃ isadded to the electrolyte as an additive in some embodiments. LiNO₃ canpassivate the surface of a lithium anode and thus reduce the shuttleeffect. FIG. 9a shows that, in the presence of LiNO₃, the initialdischarge capacity is about 1,560 mAh/g, approaching the theoreticalcapacity of sulfur. The cycling stability is similar to the sampleswithout the LiNO₃ additive. More importantly, the average coulombicefficiency increases from about 84% to over about 99% at C/5 and fromabout 86% to about 98% at C/2 (FIG. 9b ). The improvement in coulombicefficiency confirms that the LiNO₃ additive can significantly reducepolysulfides reaction at the lithium anode and thus the shuttle effect.The combination of rational design of cathode structure and electrolyteadditives can achieve high specific capacity sulfur-based cathodes withstable cycling performance and high efficiency.

By way of summary, some embodiments provide a hollow carbonnanofiber-encapsulated sulfur cathode to achieve high performancelithium-sulfur batteries. In this rational design, sulfur is selectivelycoated onto the inner wall of carbon nanofibers by utilizing an AAOtemplate. The high aspect ratio of hollow carbon nanofibers reduces therandom diffusion of polysulfides in the organic electrolyte, while thethin carbon wall allows fast transport of lithium ions. In someembodiments, a stable discharge capacity of about 730 mAh/g is retainedafter more than 150 cycles of charge/discharge at C/5. Addition of LiNO₃to the electrolyte can further improve the coulombic efficiency to about98% and about 99% at C/2 and C/5, respectively.

Synthesis of Hollow Carbon Nanofiber-Encapsulated Sulfur:

AAO membrane (Whatman, pore size of about 200 nm, thickness of about 60μm) was used as the template for making carbon nanofibers. Typically,about 120 mg AAO was placed inside an alumina boat, and about 2 mlpolystyrene (“PS”) suspended in dimethylformamide (“DMF,” about 0.1g/ml) was dropped onto the template as the carbon precursor. Thecarbonization was carried out by heating AAO/PS/DMF at about 750° C. forabout 4 hours under a slow flow of N₂ gas. After cooling down, about 15mg of carbon-coated AAO template was loaded into a small glass vial, andabout 300 μl of about 1% sulfur solution in toluene was dropped onto thetemplate. The amount of sulfur solution was controlled so that the finalloading of sulfur in the AAO template was about 1 mg. After drying, themixture was heated up to about 155° C. and kept for about 12 hours toensure sufficient sulfur diffusion into the hollow nanofibers. The AAOtemplate helped prevent sulfur coating onto the outer surface of thecarbon nanofibers. Sulfur residue sticking on the surface of thetemplate was washed away using methanol. Total sulfur loading in thesample was calculated by weighing the sample before and after sulfurinfusion. The AAO template was removed by immersing in a solution of 2 MH₃PO₄ for about 10 hours.

Characterizations:

FEI XL30 Sirion SEM with field emission guns (“FEG”) source was used forSEM characterizations. FEI Tecnai G2 F20 X-TWIN Transmission ElectronMicroscope was used for TEM characterizations. A Renishaw RM1000 Ramanmicroscope at the Extreme Environments Laboratory at Stanford Universitywas used for the Raman spectroscopy.

Electrochemical Measurement:

To evaluate the electrochemical performance of hollow carbonnanofiber/sulfur composite, 2032-type coin cells (MTI Corporation) werefabricated. The prepared samples were pressed onto aluminum substrate asthe working electrode without any binder or conductive additives.Lithium foil (Alfa Aesar) was used as the counter electrode. Theelectrolyte is about 1 M lithium bis(trifluoromethanesulfonyl)imide(“LiTFSI”) in 1,3-dioxolane and 1,2-dimethoxyethane (volume ratio ofabout 1:1). For electrolyte with LiNO₃ additive, LiNO₃ (Sigma Aldrich)was first dried at about 100° C. under vacuum over night, before beingadded to the electrolyte to reach a concentration of about 0.1 mol/L.The mass loading of sulfur in the working electrode was about 1.0mg/cm². Batteries testing were performed using a 96-channel batterytester (Arbin Instrument). The voltage range is 1.7-2.6 V vs Li/Li⁺

SEM Characterization of AAO Template:

AAO template has different morphology on the two sides (FIG. 10). Thepore sizes are about 200 nm to about 300 nm.

Raman Spectroscopy Measurement:

FIG. 11 shows Raman spectra of four samples.

X-Ray Diffraction Characterizations:

FIG. 12 shows a comparison of the XRD spectra for pristine sulfur andhollow carbon nanofiber-encapsulated sulfur. A series of peaks areobserved in XRD spectra for AAO template heated to about 780° C., whileAAO template heated to about 750° C. shows no diffraction peaks (FIG.13). This indicates a phase transition of AAO between about 750° C. andabout 780° C.

Voltage Profiles:

FIG. 14 shows charge/discharge voltage profiles of hollow carbonnanofiber-encapsulated sulfur at C/10 and C/5.

Example 2 Polymer-Encapsulated Hollow Sulfur Nanoparticles

This example describes the implementation of a monodisperse,polymer-encapsulated hollow sulfur nanoparticle cathode through ascalable, one-stage, room-temperature synthesis. The cathodeincorporates features to largely or fully address various challenges ofsulfur-based materials. The synthesis is based on a reaction betweensodium thiosulfate and hydrochloric acid in an aqueous solution in thepresence of poly(vinyl pyrrolidone) (“PVP”). The reaction can berepresented as the following:Na₂S₂O₃+2HCl→S↓+SO₂↑+NaCl+H₂O  (1)

Compared with other possible approaches for sulfur cathode synthesis,the fabrication of sulfur nanoparticles offers one or more of thefollowing advantages. 1) Neither time-consuming procedures nor hightemperatures are involved. The synthesis is carried out at about roomtemperature within about two hours in one stage. 2) The synthesis islow-cost, environmentally benign, and highly reproducible. The synthesiscan produce hollow sulfur nanoparticles with high quality on a scale ofgrams per batch. Such a synthesis also can be readily scaled up forindustrial applications. 3) The use of sulfur nanoparticles in batteryelectrodes is compatible with traditional lithium-ion batterymanufacturing techniques by allowing the use of conventional conductiveadditives, binders, and electrolytes.

FIG. 15a schematically shows the formation mechanism forpolymer-encapsulated hollow sulfur nanoparticles. PVP molecules can formhollow microspheres due to a self-assembly process, and can be used as asoft template for synthesizing hollow spheres of conductive polymers. Inaqueous solution, both the polymer backbone and the methylene groups inthe five-membered ring of PVP can allow the association of PVP moleculesthrough hydrophobic interaction, while the electronegative amide groups(dots) are effectively linked together through the hydrogen bond networkof water. Therefore, it is expected that the PVP molecules canself-assemble into a hollow spherical vesicular micelle with adouble-layer structure, having their hydrophobic alkyl backbones pointedtoward the interior of the micelle wall and the hydrophilic amide groupfacing outward (FIG. 15a ). The hydrophobic nature of sulfur promotesits preferential growth onto the hydrophobic portion of the PVPmicelles, and thus these micelles serve as a soft template to direct thegrowth of hollow sulfur nanoparticles. PVP can also absorb on a sulfurnanoparticle surface if sulfur is exposed to water during growth,forming a dense layer of polymer coating. That is, sulfur is located inthe interior of the hollow PVP wall and is isolated from the water byPVP.

FIGS. 15b-c show SEM images of the PVP-encapsulated hollow sulfurnanoparticles. These SEM images show several features of thesenanoparticles. First, the nanoparticles are substantially monodispersein size. A statistic counting over 200 sulfur nanoparticles in asynthesis batch (FIG. 18) shows that the diameters of about 95% ofnanoparticles are in the range of about 400 nm to about 460 nm. Betweendifferent batches of synthesis, a similar monodispersity is reproduciblewith the average diameter shifted slightly within the window of about400 nm to about 500 nm. Second, the nanoparticles appear to be hollow.This was also confirmed by the distinct contrast shown in the TEM image(inset of FIG. 15c ). It is noted that many of the sulfur nanoparticleshave small pores in their walls besides the large empty space or void inthe middle. This could be due to SO₂ bubbles generated during thenanoparticle synthesis accompanying with sulfur precipitation (Eq. 1).However, despite the pores inside the sulfur wall, sulfur is stilllargely or substantially isolated from the outside solution by PVP sincesulfur is hydrophobic.

To reveal PVP shells on sulfur nanoparticles, a sulfur nanoparticlesuspension is drop-casted on a silicon substrate, followed by heating invacuum and analysis under SEM. FIG. 15d shows a schematic illustratingthe sulfur subliming process of the PVP-encapsulated hollow sulfurnanoparticles. FIG. 15e-g show the corresponding SEM images of thesample before, during, and after sulfur sublimation, respectively. Aftersubstantially all of the sulfur had been sublimed (as confirmed byEnergy-dispersive X-ray analysis showing no detectable sulfur signal),the PVP shells surrounding the nanoparticles can be readily resolved(FIG. 15g ). Thermal gravimetric analysis (FIG. 19) reveals that theamount of elemental sulfur in the sample is about 70.4 wt %. From theweight percentage of sulfur, nanoparticle size, and a thickness of a PVPshell, it can be estimated that the void volume inside each nanoparticleis about 56% of the sulfur wall volume. Assuming the volume expansion islinearly dependent on the degree of lithiation, this void volume wouldallow about 70% of the theoretical capacity or about 1,170 mAh/g to beused if only inward volume expansion is considered. FIG. 15h shows thevariations in the XPS spectra of the PVP-encapsulated hollow sulfurnanoparticles and pure elemental sulfur. For pure sulfur, two peakspositioned at about 163.9 eV and about 165.1 eV can be assigned to the S2p3/2 and S 2p1/2, respectively. As to hollow sulfur nanoparticles, thesulfur peak shifts to a higher binding energy with its core levellocated at about 167 eV, and exhibits broader full widths at halfmaximum, indicating partial charge transfer of sulfur to PVP. Thisresult shows the interaction between PVP and sulfur, which cancontribute to the effective trapping of polysulfides and thus result inexcellent capacity retention.

To evaluate whether there is any volume expansion outwards of thePVP-encapsulated hollow sulfur nanoparticles after lithiation, a sulfurnanoparticle suspension is drop-casted on a piece of conductingcarbon-fiber paper (used as substrate), and then dried in vacuumovernight. A pouch cell was assembled in an argon-filled glovebox usingthe carbon-fiber paper with sulfur nanoparticles as cathode and lithiumfoil as anode. The pouch cell was discharged at a current rate of aboutC/5 to about 1.5 V, and then the voltage was held at about 1.5 V forabout 18 h. A typical two-plateau voltage profile of the sulfur cathodecan be observed (FIG. 20), indicating that lithium ions can penetratethrough the PVP shell and react with the interior sulfur duringlithiation. After the first lithiation, the carbon-fiber paper cathodefrom the cell was retrieved and washed with 1,3-dioxolane. Typical SEMimages of the PVP-encapsulated hollow sulfur nanoparticles on thecarbon-fiber paper substrate before and after lithiation are shown inFIGS. 15i-j , while FIG. 15k presents the size distribution of thesulfur nanoparticles before and after lithiation. It can be observedthat the nanoparticles after lithiation still preserved nearly aspherical shape (FIG. 15j , marked by circles), and no noticeable sizedifference was observed between the nanoparticles before and afterlithiation (average diameter: about 483 nm (before) and about 486 nm(after)). This indicates that sulfur expands inwardly into the hollowspace, and the polymer shell is mechanically rigid enough to impedeoutward expansion or breakage. Thus, the polymer shell can effectivelyimpede polysulfides from diffusing into an electrolyte.

The inward expansion of PVP-encapsulated hollow sulfur nanoparticlesduring lithiation opens up opportunities for high performancesulfur-based battery cathodes. To test electrochemical performance, Type2032 Coin cells were assembled using a metallic lithium foil as anode.LiNO₃ was added to an electrolyte as an additive to passivate thelithium anode surface. The specific capacities were calculated based onsulfur mass alone.

FIG. 16a shows the typical discharge-charge voltage profiles of thecells made from the PVP-encapsulated, hollow sulfur nanoparticles atdifferent current rates (C/10, C/5 and C/2, where 1 C=1,673 mA/g) in thepotential range of 2.6-1.5 V at room temperature. At C/10, a capacity ofabout 1,179 mAh/g can be obtained, consistent with the estimation ofcapacity based on the available internal available space inside hollownanoparticles. At higher discharge rates of C/5 and C/2, the electrodedelivered a capacity of about 1,018 mAh/g and about 990 mAh/g,respectively. The discharge profiles of all three current densities werecharacterized by a two-plateau behavior of a typical sulfur cathode.FIG. 16b shows the cycling performances of the cells made from thePVP-encapsulated hollow sulfur nanoparticles at C/5 rate for 300 cycles.The discharge capacity exhibited a gradual increase during the firstseveral cycles, indicating an activation process for the electrodes.This activation may be due to the polymer coating on the sulfurnanoparticle surface, and may relate to an amount of time for theelectrolyte to wet an outer surface of the nanoparticles and becomeelectrochemically active. At C/5 rate, an initial capacity of about 792mAh/g was measured. After several cycles of activation, the dischargecapacity reached its highest, about 1,018 mAh/g. A capacity of about 931mAh/g was retained after 100 cycles of charge/discharge, showingexcellent capacity retention of about 91.5% (of its highest dischargecapacity of about 1,018 mAh/g). A reversible capacity of about 790 mAh/gwas still retained after 300 cycles, corresponding to a capacityretention of about 77.6% of its highest capacity, and corresponding to adecay of about 7.8% per 100 cycles. The average Coulombic efficiency ofthe cell at C/5 rate for 300 cycles is about 98.08% (FIG. 16b ).

When discharged/charged at C/2 rate (FIG. 16c ), the cell also exhibitsexcellent cycling stability. After reaching its highest capacity, thedischarge capacity stabilized at about 905 mAh/g after 10 more cycles(at the 14^(th) cycle). A discharge capacity of about 857 mAh/g andabout 773 mAh/g can be obtained after 100 and 300 cycles, correspondingto a capacity retention of about 94.7% and about 85.4% of its stabilizedcapacity at the 14^(th) cycle, respectively. After 500 and 1,000 cycles,the cell delivered a reversible discharge capacity of about 727 mAh/gand 535 mAh/g, respectively, corresponding to a capacity retention ofabout 80.3% and about 59.1% (of its stabilized capacity at the 14^(th)cycle). The capacity decay was as low as about 0.04% (about 0.37 mAh/g)per cycle. The cell also maintained a high Coulombic efficiency evenafter 1,000 cycles (FIG. 16c ), and the average over 1,000 cycles isabout 98.5%.

The excellent cycling performance of the hollow sulfur nanoparticles wasreproducible over many coin cells. Another example of theelectrochemical performance of the hollow sulfur nanoparticle electrodeis demonstrated in FIG. 16d . The cell reached its highest capacity ofabout 1,099 mAh/g after 18 cycles at C/5 rate, and showed a stablereversible capacity of about 1,026 mAh/g after 36 cycles. Furthercycling at different rates (C/2 and 1 C, each for 10 cycles) showed areversible capacity of about 800 mAh/g at C/2 rate and about 674 mAh/gat 1 C rate (a typical discharge/charge curve at 1 C rate is shown inFIG. 21). When the cell was discharged at C/5 rate again, a reversiblecapacity of about 989 mAh/g can be obtained after 10 more cycles,suggesting the high stability of the electrode. Even after another roundof cycling at various current rates, a reversible capacity of about 953mAh/g can still be retained at C/5 rate after 100 cycles, indicatingsuperior capacity reversibility and good rate performance.

For battery materials with a relatively large volume change, theassociated volume change propagating to the macroscopic scale of thewhole electrode can be a challenge. This macroscopic expansion problemcan be overcome via the PVP-encapsulated hollow sulfur nanoparticlessince the volume expansion is mitigated locally at each particle towardan inside hollow space. FIG. 17a presents a schematic showing that thewhole electrode thickness undergoes little or no change. FIG. 17b showstypical SEM images of the cross-sections of the cathode before and after20 charge/discharge cycles (at C/5 rate, fully discharged (lithiated)state). FIG. 17d shows the electrode thickness at 20 different locationsfor the cross-sections of the pristine electrode and the electrode aftercycling, showing no noticeable signs of volume expansion at the wholeelectrode level. These results are noteworthy for the design of a fullbattery.

By way of summary, substantially monodisperse, polymer-encapsulatedhollow sulfur nanoparticles are synthesized through a cost-effective,one-stage method in aqueous solution at room temperature. This exampledemonstrates excellent performance of battery electrodes formed of thesehollow nanoparticles. As a highlight, battery electrodes exhibitexcellent cycle life at or beyond 500 cycles with about 80% or morecapacity retention, which is a standard industrial specification forportable electronics.

Methods:

For the synthesis of PVP-encapsulated hollow sulfur nanoparticles, about50 mL of about 40 mM sodium thiosulfate (Na₂S₂O₃, Aldrich) aqueoussolution was mixed with about 50 mL of about 0.2 M PVP (MW of about55,000, Aldrich) at room temperature. Then, about 0.2 mL of concentratedhydrochloric acid (HCl) was added to the Na₂S₂O₃/PVP solution undermagnetic stirring. After the reaction had proceeded at room temperaturefor about 2 h, the solution was centrifuged at about 8,000 rpm for about10 min to isolate the precipitate. In the washing process, theprecipitate was washed with about 0.8 M of PVP aqueous solution once andcentrifuged at about 6,000 rpm for about 15 min.

For SEM and TEM characterization, SEM images were taken using FEI XL30Sirion SEM operated at an accelerating voltage of about 5 kV. TEMimaging was performed on a FEI Tecnai G2 F20 X-TWIN TEM operated atabout 200 kV.

For electrochemical measurement, the PVP-encapsulated hollow sulfurnanoparticle powder was mixed with Super-P carbon black andpolyvinylidene fluoride (PVDF) binder, with mass ratio of about60:25:15, in N-Methyl-2-pyrrolidone (NMP) solvent to produce anelectrode slurry. The slurry was coated onto an aluminum foil currentcollector using doctor blade and then dried to form the workingelectrode. The typical mass loading of active sulfur was in the range ofabout 0.8-1.8 mg/cm². 2032-type coin cells (MTI Corporation) werefabricated using the working electrode and lithium metal foil as thecounter electrode. The electrolyte was about 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and about 0.1 M LiNO₃ in 1,3-dioxolaneand 1,2-dimethoxyethane (volume ratio of about 1:1). The coin cells wereassembled inside an argon-filled glovebox. Galvanostatic measurementswere made using MTI battery analyzers. The specific capacities were allcalculated based on the mass of active sulfur.

Calculation of the Void Volume Inside Each Nanoparticle:

Density of sulfur: ρ_(S)=2 g/cm³

Density of PVP: ρ_(PVP)=1.2 g/cm³

The thickness of PVP shell on the sulfur particle (based on FIG. 2g ):28.5 nm.

The diameter of the PVP-encapsulated sulfur nanoparticles: D=483 nm.

The outer diameter of the sulfur wall is d_(out)=483−28.5×2=426 nm

The volume of PVP shell (V_(PVP)) is:

$\begin{matrix}{V_{PVP} = {4\text{/}3 \times \pi \times \left( {\left( {D\text{/}2} \right)^{3} - \left( {d_{out}\text{/}2} \right)^{3}} \right)}} \\\left. {= {{4\text{/}3 \times \pi \times \left( {483\text{/}2} \right)^{3}} - \left( {426\text{/}2} \right)^{3}}} \right)\end{matrix}$

The weight of PVP (m_(PVP)) is:m _(PVP) =V _(PVP)×ρ_(PVP)

Since the weight of PVP is 30% of the weight of the PVP-encapsulatedsulfur particles,

So the volume of sulfur (Vs) is

$\begin{matrix}{{Vs} = {m_{PVP}\text{/}30\% \times 70\%\text{/}\rho_{S}}} \\{= {2.59 \times 10^{7}\mspace{14mu}{nm}^{3}}}\end{matrix}$

So the inner diameter of the sulfur wall (d_(m)) is:Vs=4/3×π×((d _(out)/2)³−(d _(m)/2)³)d _(in)=303 nm

Thus, the void volume inside each hollow nanoparticle is represented as:(d _(in)/2)³/((d _(out)/2)³−(d _(in)/2)³)=56%  (2)

Size Distribution of Hollow Sulfur Nanoparticles:

FIG. 18 shows a size distribution of hollow sulfur nanoparticles, basedon counting over 200 nanoparticles from SEM images.

Thermal Gravimetric Analysis:

FIG. 19 shows a TGA curve of as-prepared PVP-encapsulated hollow sulfurnanoparticles recorded in the range of 40-400° C. in argon at a heatingrate of about 2° C./min, showing that the amount of elemental sulfur inthe sample is about 70.4 wt %.

Voltage Profile:

FIG. 20 shows a voltage profile of a pouch cell assembled in anargon-filled glovebox using a carbon-fiber paper with PVP-encapsulatedhollow sulfur nanoparticles as cathode and lithium foil as anode.

Discharge/Charge Profile:

FIG. 21 shows a typical discharge/charge profiles of a cell made fromPVP-encapsulated hollow sulfur nanoparticles at 1 C rate.

Example 3 Sulfur-TiO₂ Yolk-Shell Nanostructures

This example describes the implementation of a sulfur-TiO₂ yolk-shellnanoarchitecture for stable and prolonged cycling over 1,000charge/discharge cycles in lithium-sulfur batteries. An advantage of theyolk-shell morphology lies in the presence of an internal void space toaccommodate a relatively large volumetric expansion of sulfur duringlithiation, thus preserving a structural integrity of a shell tomitigate against polysulfide dissolution. In comparison with bare sulfurand sulfur-TiO₂ core-shell nanoparticles, the yolk-shell nanostructuresare found to exhibit a high capacity retention due to the effectivenessof the intact TiO₂ shell in restricting polysulfide dissolution. Usingthe yolk-shell nanoarchitecture, an initial specific capacity of about1,030 mAh/g at ½ C rate and a Coulombic efficiency of about 98.4% over1,000 cycles was achieved. Moreover, the capacity decay at the end of1,000 cycles was found to be as small as about 0.033% per cycle (3.3%per 100 cycles).

The sulfur-TiO₂ yolk-shell morphology was experimentally realized asshown schematically in FIG. 22a . First, substantially monodispersesulfur nanoparticles were prepared using the reaction of sodiumthiosulfate with hydrochloric acid (FIG. 25). The sulfur nanoparticleswere then coated with TiO₂ through controlled hydrolysis of a sol-gelprecursor, titanium diisopropoxide bis(acetylacetonate), in an alkalineisopropanol/aqueous solution, resulting in the formation of sulfur-TiO₂core-shell nanoparticles (FIG. 26; a TEM image was taken immediatelyafter an electron beam was turned on to avoid sublimation of sulfurunder the beam). This was followed by partial dissolution of sulfur intoluene to create an empty space between the sulfur core and the TiO₂shell, resulting in the yolk-shell morphology. The ability of toluene todiffuse through the TiO₂ shell to partially dissolve sulfur indicatesits porous nature. A SEM image in FIG. 22b shows relatively uniformspherical nanoparticles of about 800 nm in size. A TEM image in FIG. 22c, taken immediately after the electron beam was turned on, shows sulfurnanoparticles encapsulated within TiO₂ shells (about 15 nm thick) withinternal void space. Due to the two-dimensional projection nature of TEMimages, the void space appears as an empty area or an area of lowerintensity depending on the orientation of the particles (FIG. 22c ). TheTiO₂ in the yolk-shell nanostructures were determined to be amorphoususing X-ray diffraction (FIG. 27).

Next, the effectiveness of the yolk-shell morphology was investigated interms of accommodating the volume expansion of sulfur and restrictingpolysulfide dissolution. The sulfur-TiO₂ yolk-shell nanostructures weredrop-cast onto conducting carbon-fiber papers to form workingelectrodes, and pouch cells were assembled using lithium foil as thecounter electrode. The cells were discharged at 0.1 C rate (1 C=1,673mA/g) to a voltage of 1.7 V vs. Li+/Li, during which a capacity of about1,110 mAh/g was attained (FIG. 28), and the voltage was maintained forover 20 h. The as-obtained discharge profile shows the typicaltwo-plateau behavior of sulfur cathodes, indicating the conversion ofelemental sulfur to long-chain lithium polysulfides (Li₂S_(n), 4≤n≤8) atabout 2.3 V, and the subsequent formation of Li₂S₂ and Li₂S at about 2.1V (FIG. 28). After the lithiation process, the contents of the cells(cathode, anode, and separator) were washed with 1,3-dioxolane solutionfor further characterization. This polysulfide-containing solution wasthen oxidized with concentrated HNO₃ and diluted with deionized waterfor analysis of sulfur content using inductively coupled plasmaspectroscopy (“ICP”). For comparison, electrode materials were alsoprepared using bare sulfur and sulfur-TiO₂ core-shell nanoparticles andsubjected to the same treatment.

There was little change in morphology and size distribution of thesulfur-TiO₂ yolk-shell nanostructures before and after lithiation (FIGS.23a-c ). A TEM image of a lithiated yolk-shell nanostructure shows astructurally intact TiO₂ coating (FIG. 23d ), indicating the ability ofthe yolk-shell morphology in accommodating the volume expansion ofsulfur. The presence of lithiated sulfur and TiO₂ in the yolk-shellnanostructure was confirmed using energy-dispersive X-ray spectroscopyand electron energy loss spectroscopy (FIG. 23e ). In the case of baresulfur and sulfur-TiO₂ core-shell nanoparticles, random precipitation ofirregularly-shaped Li₂S₂ and Li₂S particles was observed on theelectrodes due to dissolution of lithium polysulfides into theelectrolyte (FIG. 29). ICP analysis showed a loss of about 81% and about62% of the total sulfur mass into the electrolyte for the bare sulfurand sulfur-TiO₂ core-shell nanoparticles, respectively. In comparison,about 19% of the total sulfur mass was found to be dissolved in theelectrolyte in the case of the yolk-shell nanostructures, whichindicates the effectiveness of the intact TiO₂ shell in restrictingpolysulfide dissolution.

To further evaluate the electrochemical cycling performance of thesulfur-TiO₂ yolk-shell nanoarchitecture, 2032-type coin cells werefabricated. The working electrodes were prepared by mixing theyolk-shell nanostructures with conductive carbon black andpolyvinylidene fluoride binder in N-methyl-2-pyrrolidinone to form aslurry, which was then coated onto an aluminum foil and dried undervacuum. Using a lithium foil as the counter electrode, the cells werecycled from 1.7-2.6 V vs. Li+/Li. The electrolyte used was lithiumbis(trifluoromethanesulfonyl)imide in 1,2-dimethoxyethane and1,3-dioxolane, with LiNO₃ (1 wt %) as an additive to passivate a surfaceof the lithium anode. Specific capacity values were calculated based onthe mass of sulfur, which was determined using TGA (FIG. 30). The sulfurcontent was found to be about 71 wt % in the yolk-shell nanostructures,accounting for about 53 wt % of the electrode mix, and with a typicalsulfur mass loading of about 0.4-0.6 mg/cm². The contribution of TiO₂ tothe total capacity is relatively small in the voltage range of thisexample.

The sulfur-TiO₂ yolk-shell nanoarchitecture exhibited stable cyclingperformance over 1,000 charge/discharge cycles at ½ C rate (1 C=1,673mA/g) as displayed in FIG. 24a (see also FIG. 31). After an initialdischarge capacity of about 1,030 mAh/g, the yolk-shell nanostructuresachieved capacity retentions of about 88%, about 87%, and about 81% atthe end of 100, 200, and 500 cycles, respectively (FIGS. 24a-b ).Moreover, after prolonged cycling over 1,000 cycles, the capacityretention was found to be about 67%, which corresponds to a smallcapacity decay of about 0.033% per cycle (about 3.3% per 100 cycles).The average Coulombic efficiency over the 1,000 cycles was calculated tobe about 98.4% (FIG. 24a ), which shows little shuttle effect due topolysulfide dissolution. In comparison, cells based on bare sulfur andsulfur-TiO₂ core-shell nanoparticles suffered from rapid capacity decay,showing capacity retentions of about 48% and about 66% respectivelyafter 200 cycles (FIG. 24b ), indicating a greater degree of polysulfidedissolution into the electrolyte.

Next, the sulfur-TiO₂ yolk-shell nanostructures were subjected tocycling at various C-rates to evaluate their robustness (FIGS. 24c-d ).After an initial discharge capacity of about 1,215 mAh/g at 0.2 C rate,the capacity was found to stabilize at about 1,010 mAh/g. Furthercycling at 0.5 C, 1 C, and 2 C showed high reversible capacities ofabout 810 mAh/g, about 725 mAh/g, and about 630 mAh/g, respectively(FIGS. 4c-d ). When the C-rate was switched abruptly from 2 C to 0.2 Cagain, the original capacity was largely recovered (FIG. 24c ),indicating robustness and stability of the cathode material. Moreover,there was little change in the thickness of the cathode before and after70 cycles at these various C-rates (FIG. 32), which further confirms theability of the yolk-shell nanostructures in accommodating the volumeexpansion of sulfur.

There are at least two characteristics of a yolk-shell design thatimpart the sulfur-TiO₂ nanostructures with stable cycling performanceover 1,000 charge/discharge cycles. First, sufficient empty space ispresent to allow for volume expansion of sulfur. Using image processingsoftware on the yolk-shell nanostructures (FIG. 22c ), sulfur wasdetermined to occupy about 62% of the volume within the TiO₂ shell,which corresponds to about 38% internal void space. This value issupported by TGA of the relative wt % of sulfur vs. TiO₂ (FIG. 30), fromwhich the volume of empty space in the yolk-shell nanostructures wasestimated to be about 37%. This volume of void space can accommodateabout 60% volume expansion of the sulfur present within the shell,allowing for about 1,250 mAh/g, namely about 75% of the maximumtheoretical capacity of sulfur (assuming volume expansion is linearlydependent on the degree of lithiation). Experimentally, a maximumdischarge capacity of about 1,215 mAh/g has been achieved (FIG. 24c );therefore, there is sufficient void space for volume expansion withoutcausing the shell to crack and fracture. Second, the intact TiO₂ shellis effective in mitigating against polysulfide dissolution. The abilityof toluene to diffuse through the TiO₂ shell to partially dissolvesulfur (FIG. 22a ) indicates the porous (<2 nm) nature of the shell,which is typical of amorphous TiO₂ prepared using sol-gel methods. Thestable cycling performance demonstrated in this example indicates thatthe TiO₂ shell is effective in restricting polysulfide dissolution dueto its small pore size and the presence of hydrophilic Ti—O groups thatcan bind favorably with polysulfide anions.

By way of summary, this example demonstrates the design of a sulfur-TiO₂yolk-shell nanoarchitecture for long cycling capability over 1,000charge/discharge cycles, with a capacity decay as small as about 0.033%per cycle. Compared to bare sulfur and sulfur-TiO₂ core-shellcounterparts, the yolk-shell nanostructures exhibited the highestcapacity retention due to the presence of internal void space toaccommodate the volume expansion of sulfur during lithiation, resultingin an intact shell to restrict polysulfide dissolution.

Synthesis of Sulfur-TiO₂ Yolk-Shell Nanostructures:

First, sulfur nanoparticles were synthesized by adding concentrated HCl(0.8 mL, 10 M) to an aqueous solution of Na₂S₂O₃ (100 mL, 0.04 M)including a low concentration of polyvinylpyrrolidone (PVP, MW of about55,000, 0.02 wt %). After reaction for about 2 h, the sulfurnanoparticles (100 mL) were washed by centrifugation and redispersedinto an aqueous solution of PVP (20 mL, 0.05 wt %). For TiO₂ coating,the solution of sulfur nanoparticles (20 mL) was mixed with isopropanol(80 mL) and concentrated ammonia (2 mL, 28 wt %). Titaniumdiisopropoxide bis(acetylacetonate) (50 mL, 0.01 M in isopropanol) wasthen added in five portions (5×10 mL) at about half-hour intervals.After reaction for about 4 h, the solution of sulfur-TiO₂ core-shellnanoparticles was washed by centrifugation to remove freely-hydrolyzedTiO₂, followed by redispersion into deionized water (20 mL). To preparethe sulfur-TiO₂ yolk-shell nanostructures, the solution of core-shellparticles (20 mL) was stirred with isopropanol (20 mL) and toluene (0.4mL) for about 4 h to achieve partial dissolution of sulfur. Theas-synthesized sulfur-TiO₂ yolk-shell nanostructures were then recoveredusing centrifugation and dried under vacuum overnight.

Characterization:

SEM and TEM images were taken using a FEI XL30 Sirion SEM (acceleratingvoltage 5 kV) and a FEI Tecnai G2 F20 X-TWIN (accelerating voltage 200kV), respectively. Elemental analysis was performed usingenergy-dispersive X-ray spectroscopy and electron energy lossspectroscopy equipped in the TEM device. X-ray diffraction patterns wereobtained on a PANalytical X'Pert Diffractometer using Cu Kα radiation.TGA was carried out using a Netzsch STA 449 at a heating rate of about2° C./min under argon atmosphere. ICP optical emission spectroscopy wasperformed using a Thermo Scientific ICAP 6300 Duo View Spectrometer.

Electrochemical Measurements:

To prepare working electrodes, various sulfur-based materials were mixedwith carbon black (Super P) and polyvinylidene fluoride (PVDF) binder(75:15:10 by weight) in N-methyl-2-pyrrolidinone (NMP) to form a slurry.This slurry was then coated onto an aluminum foil using doctor blade anddried under vacuum to form a working electrode. 2032-type coin cellswere assembled in an argon-filled glove box using a lithium foil as thecounter electrode. The electrolyte used was a solution of lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI, 1 M) in 1:1 v/v1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) including LiNO₃ (1 wt%). Galvanostatic cycling was carried out using a 96-channel batterytester (Arbin Instruments) from 1.7-2.6 V vs. Li+/Li. Specific capacityvalues were calculated based on the mass of sulfur in the samples, whichwas determined using TGA (FIG. 30). The sulfur content was found to beabout 71 wt % in the yolk-shell nanostructures, accounting for about 53wt % of the electrode mix, with a typical sulfur mass loading of about0.4-0.6 mg/cm².

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

What is claimed is:
 1. A method of forming a cathode material,comprising: providing a sulfur-based nanostructure; coating thenanostructure with an encapsulating material to form a shell surroundingthe nanostructure; and removing a portion of the nanostructure throughthe shell to form a void within the shell, with a remaining portion ofthe nanostructure disposed within the shell.
 2. The method of claim 1,wherein the shell includes a wall that is porous.
 3. The method of claim1, wherein removing the portion of the nanostructure is performed bypartial dissolution of the nanostructure in a solvent.
 4. The method ofclaim 1, wherein a volume of the void is at least 1/3 of an internalvolume of the shell.
 5. The method of claim 1, wherein a volume of thevoid is at least 2/3 of an internal volume of the shell.